Utilization of the Doppler effect in flowmeters for measuring the velocity of flow of such fluids as blood in such conduits as blood vessels are well known. Examples of such flowmeters are described in U.S. Pat. Nos. 3,732,532 (Flaherty et al); 3,766,517 (Fahrback); and 3,861,211 (Dewan). However, such flowmeters employ ultrasonic wave energy, which is not satisfactory for velocity measurements of solid materials carried by high temperature gases at elevated pressures through pipes or chemical process reactors, since such wave energy cannot effectively penetrate conventional sight ports used with pressure vessels. Although the Doppler effect has been utilized in intrusion detection systems which employ microwave electromagnetic wave energy, see, e.g., U.S. Pat. No. 3,845,461 (Foreman), the use of Doppler radar techniques to measure the flow of particulates has been limited. An example of a system used to monitor transported materials in pneumatic conveying applications is described in the following articles by Harris: J. Harris, Proc. Symp., "The Measurement of Pulsating Flow," p. 2-3, April 1970, Institute of Measurement and Control; "Flow Measurement Using a Microwave Doppler Meter," International Conference on Modern Developments in Flow Measurement, A.E.R.E., Harwell, p. 21-23, (September 1971; "Flow Measurement Using Microwave Radar Techniques," Powder Technology, p. 85-89, Vol. 6, 1972.
A major disadvantage of such conventional microwave Doppler flowmeters is distortion of measurements due to local noise fluctuations. Attempts at reducing the effect of noise have involved the use of multiple antennas and a signal correlating wave guide to cancel, at microwave frequencies, the local noise fluctuations. As a consequence, conventional microwave Doppler flowmeters require microwave filtering and amplification equipment.
A further major disadvantage of prior art flowmeters concerns the calibration difficulties associated therewith when the particulate flow to be measured cannot be observed.